This invention relates to a method of protecting ground-supported, liquid storage metal tanks from potentially damaging earthquake ground shaking. It also includes an energy-dissipating seismic anchor especially designed to limit the transfer of seismic energy into a liquid storage tank and hence reduce the forces experienced by the tank.
Ground-supported cylindrical tanks are used to store a variety of liquids, e.g., water for drinking and fire-fighting, crude oil, wine and liquefied natural gas (LNG). Tanks are critical components of modern industrial facilities and lifeline systems, and must be designed to withstand safely the earthquakes to which they are subjected. The failure of such systems may lead to environmental hazard, loss of valuable contents, and disruption of fire-fighting efforts following destructive earthquakes.
Ground-supported liquid-storage metal tanks are designed to be either fully (rigidly) anchored or unanchored at their base (API, "Welded Steel Tanks for Oil Storage," API Standard 650, 9th Ed., American Petroleum Institute, Washington, D.C., 1993; AWWA, "Welded Steel Tanks for Water Storage," AWWA D100, American Water Works Association, Denver, Colo., 1996). When subjected to severe earthquake ground shaking, fully anchored tanks develop large base shear and overturning moment due to hydrodynamic action and impose high demands on their base anchorage system and foundation. High stresses in the vicinity of poorly detailed anchors can tear the tank wall, and large base shear can overcome friction between the base and the foundation, causing the tank to slide.
When subjected to ground shaking stronger than design, a traditionally anchored tank experiences inelastic stretching or pulling of the anchor bolts. The energy loss due to the inelastic action of the anchor bolts is, however, quite small because the bolts act in tension only--they do not exhibit a cyclic load path capable of dissipating energy in each vibration cycle. Anchor bolts that are not detailed properly can suddenly break or slip leading to a sharp increase in base uplift and associated responses, such as plastic rotations in the base plate, radial separation between the base plate and the foundation, and hoop stress in the tank wall.
Tanks that are unanchored at their base experience partial base uplifting when subjected to strong ground shaking. Increased flexibility associated with base uplifting reduces the hydrodynamic pressures, hence the base shear and overturning moment. However, due to reduced contact of the wall with the foundation, the axial compressive stress increases--leading in severe cases to buckling of the wall.
Unanchored tanks supported directly on flexible soils experience smaller axial compressive stress and are, therefore, less prone to buckling damage as compared to unanchored tanks supported on concrete foundations. However, such tanks can undergo large base uplift, foundation penetration, plastic rotation at plate boundary, hoop compressive stress in the wall, and radial separation between the plate and foundation. Large uplifts can damage the piping connections to the wall, and large foundation penetrations can cause uneven and permanent settlement of the wall due to nonlinear soil response. Several cycles of large plastic rotations can rupture the plate-shell junction leading to a loss of tank's content.
Unsatisfactory performance of both anchored and unanchored tanks when subjected to truly strong ground shaking stems directly from their inability to dissipate large amount of seismic energy. Methods of seismic strengthening of tanks have been proposed in U.S. Pat. No. 3,977,140 to Matsudaira et al., U.S. Pat. No. 4,249,352 and U.S. Pat. No. 4,267,676 to Marchaj and U.S. Pat. No. 4,697,395 to Peek. However, these methods do not increase the energy-dissipation capacity of cylindrical metal tanks.
Methods of base isolation have also been proposed to improve the seismic performance of tanks. Kelly, T. E., and Mayes, R. L. (1989), "Seismic isolation of storage tanks," Proc., Sessions Related to seismic Engrg. at Structures Congress '89; C. A. Kircher and A. K. Chopra, eds., ASCE, New York, N.Y., p.p. 408-417; Taijirian, F. E. (1993), "Seismic isolation of critical components and tanks," Proc., ATC-17-1 Seminar on Seismic Isolation, Passive Energy Dissipation, and Active Control. San Francisco, Calif., Vol. 1, 233-244. In these methods, the tank is supported on a large concrete mat, which, in turn, is supported on several isolation bearings. Although suitable for tanks for which a concrete mat supported above the ground already exists, these methods of isolation are unsuitable for numerous other tanks that are supported directly on the ground.